help with Smiths ETU7000 please!?

My boiler, which is only used to heat water as we dont have central heating has decided not work - the time is correct but the word 'on' is flashing. If you press the button that usually heats the water up if you need it at a different time, the 'on' changes to 'off' and nothing happens.
I will try and reset it following the instructions i found on here but does anyone have any ideas why this would be? We rent our house and there are no instructions for the boiler!!
Thanks loads - Kate